Query by image

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on computer storage media, for processing queries made up of images. In one aspect, a method includes indexing images by image descriptors. The method further includes associating descriptive n-grams with the images. In another aspect, a method includes receiving a query, identifying text describing the query, and performing a search according to the text identified for the query.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims priority to,U.S. patent application Ser. No. 12/960,413, for Query By Image, filedon Dec. 3, 2010, which claims the benefit under 35 U.S.C. §119 of U.S.Provisional Application Ser. No. 61/266,503, filed on Dec. 3, 2009,which is incorporated herein by reference.

BACKGROUND

This specification relates to image search using query images.

Search engines aim to identify resources (e.g., images, audio, video,web pages, text, or documents) that are relevant to a user's needs andto present information about the resources in a manner that is mostuseful to the user. Search engines return a set of search results inresponse to a user submitted query. For example, in response to an imagesearch text query (i.e., a query to identify image resources), thesearch engine returns a set of search results identifying imageresources responsive to the query (e.g., as a group of thumbnailrepresentations of the image resources). Typically, queries are textqueries, e.g., queries having one or more search terms or query phrases.

SUMMARY

This specification describes technologies relating to image search usingquery images.

In general, one aspect of the subject matter described in thisspecification can be embodied in methods that include the actions ofreceiving a group of images, each image having associated metadata;building an initial index using local descriptors generated for eachimage of the group of images; generating a set of matching images foreach image using the initial index; using the set of matching images todetermine pairwise image affinity and final image selection; calculatinga set of n-grams and corresponding n-gram scores for each image usingimage affinity and image metadata; and generating a final index usingimages for the final image selection and including corresponding n-gramsfor each image. Other embodiments of this aspect include correspondingsystems, apparatus, and computer programs recorded on computer storagedevices, each configured to perform the operations of the methods.

These and other embodiments can each optionally include one or more ofthe following features. The method can further include, for each image,generating local descriptors for each of multiple regions of interestwithin the image. The matched images can have a score that is correlatedto a visual similarity between matching images. Using the set ofmatching images to determine pairwise image affinity includes generatingan undirected graph using the set of matching images and wheredetermining the pairwise image affinity comprises calculating a maximumaffinity path between any two images in the graph. Determining a finalimage selection can include identifying a value for a selection flag ofeach image in the graph according to a calculated maximum yield over theentire graph. The n-grams for each image and the associated n-gramscores can be calculated using the image metadata and image affinity.The image metadata can include a number of clicks for the imageassociated with a query phrase.

In general, another aspect of the subject matter described in thisspecification can be embodied in methods that include the actions ofreceiving an image as a query; deriving a textual query corresponding tothe received image using n-grams associated with similar images to thereceived image; obtaining search results responsive to the textualquery; and presenting one or more of the obtained search results. Otherembodiments of this aspect include corresponding systems, apparatus, andcomputer programs recorded on computer storage devices, each configuredto perform the operations of the methods.

These and other embodiments can each optionally include one or more ofthe following features. The image can be received from a mobile device.Deriving the textual query corresponding to the image can includeobtaining a group of similar images for a particular image, each similarimage associated with one or more n-grams; determining an overall scorefor one or more of the n-grams; and selecting one of the n-grams as adescription of the particular image. The search results can be imagesearch results.

In general, another aspect of the subject matter described in thisspecification can be embodied in methods that include the actions ofobtaining a group of similar images for a particular image, each similarimage associated with one or more n-grams; determining an overall scorefor one or more of the n-grams; and selecting one of the n-grams as adescription of the particular image according to the overall scores forthe one or more n-grams. Other embodiments of this aspect includecorresponding systems, apparatus, and computer programs recorded oncomputer storage devices, each configured to perform the operations ofthe methods.

These and other embodiments can each optionally include one or more ofthe following features. Obtaining a group of similar images for aparticular image can include querying an index that associates imageswith descriptors. The method can further include storing a databaseassociating each image with one or more n grams. Each n-gram associatedwith an image can have an image-specific score.

Determining an overall score for an n-gram can include combining eachimage-specific score for the n-gram. The overall score for an n-gram canbe based on a sum of the image-specific scores for the n-gram divided bya total number of images associated with the n-gram. The method canfurther include filtering one or more of the n-grams according to ablacklist of terms. The method can further include filtering one or moreof the n-grams to remove n-grams that contain stop words.

Selecting one of the n-grams can include selecting a current best n-gramfrom one or more 1-grams in the one or more n-grams, where the currentbest n-gram has a highest overall score of all of the one or more1-grams in the one or more n-grams; considering each other highestoverall scoring n-gram of all of one or more n-grams at each order ofn-grams greater than 1 in increasing order, and determining, for eachhighest overall scoring n-gram at a given order whether to update thecurrent best n-gram to be the highest overall scoring n-gram of theorder, where the current best n-gram is updated when the highest overallscoring n-gram of the order has a higher overall score than a scoreassociated with the current best n-gram or when the highest overallscoring n-gram is a superstring of the current best n-gram and has ascore that satisfies an acceptance threshold. The method can furtherinclude updating the current best n-gram to be the determined highestoverall scoring n-gram of the order when it is determined that thecurrent best n-gram should be updated. Updating the current best n-gramcan include associating a score with the updated current best n-gram,wherein the score is the overall score of the updated current bestn-gram when the updated current best n-gram has an overall score that ishigher than the score associated with the previous best n-gram, andotherwise is the score of the previous best n-gram.

In general, another aspect of the subject matter described in thisspecification can be embodied in a data processing apparatus, comprisinga first memory store, the first memory store storing a tree, the firsttree having a root node, a plurality of child nodes, and leaf nodesdescending from the child nodes; and a plurality of second memorystores, each second memory store storing one of a plurality of secondtrees, each second tree being a tree having a root node corresponding toone of the leaf nodes of the first tree, and each second tree includinga plurality of child nodes and leaf nodes descending from its childnodes, wherein each leaf node in each second tree corresponds to a queryvector.

These and other embodiments can each optionally include one or more ofthe following features. The first memory store can be a random accessmemory in a first computer, and each second memory store can be a randomaccess memory on a respective second computer of a plurality of secondcomputers that correspond to the plurality of second memory stores. Thefirst tree can be a binary tree having a depth d, and the plurality ofsecond trees can consist of 2^(d) second trees.

In general, another aspect of the subject matter described in thisspecification can be embodied in methods that include the actions ofselecting a proper subset of elements from a set of elements; building afirst tree using the proper subset of elements, the first tree having aroot node, a plurality of child nodes and leaf nodes descending from thechild nodes; associating each element in the set of elements with acorresponding child node so that each child node has a unique set ofassociated elements from the set of elements; and for each child node,building a second tree using the associated elements associated with thechild node, the second tree having a root node, a plurality of childnodes and leaf nodes descending from the child nodes, wherein each leafnode in each second tree corresponds to a query vector. Otherembodiments of this aspect include corresponding systems, apparatus, andcomputer programs recorded on computer storage devices, each configuredto perform the operations of the methods.

These and other embodiments can each optionally include one or more ofthe following features. The method can further include receiving a queryvector; identifying a proper subset of child nodes of the first treethat are most similar to the query vector; providing the query vector toeach of the second trees that correspond to the proper subset of childnodes; for each second tree, identifying a proper subset of child nodesof the second tree that are most similar to the query vector, whereineach child node of the second tree corresponds to an indexed vector; andranking the indexed vectors that are identified for each second treebased on the similarity of each indexed vector to the query vector.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. An image index is generated that provides forefficient and accurate retrieval of images responsive to a given imageas a query. Users can submit queries as images, rather than as text.This allows users to submit queries, for example, to find otherinformation relevant to a particular image, even if the user is not sureof the best way to describe the image. This also allows users to submitqueries in situations where entering text would be cumbersome, forexample, on a mobile device keyboard or touch input interface.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example system for generating asearchable index of image content.

FIG. 2 is a flow chart showing an example method for generating asearchable index of image content.

FIG. 3 is a block diagram of an example distributed kd-treeinfrastructure.

FIG. 4 is a flow chart showing an example method for constructing adistributed search structure.

FIG. 5 is a flow chart showing an example method for an approximatenearest neighbor search on the distributed search structure.

FIG. 6 illustrates an example search system for providing searchresults.

FIG. 7 is a flow chart of an example method for obtaining search resultsresponsive to an image query received from a user.

FIG. 8 is a flow chart of an example method for selecting a textualdescription of an image.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a system 100 for generating a searchableindex of image content. The system 100 includes an image database 102.The image database 102 includes a collection of images. The collectionof images includes images collected from various resources, for example,web pages and other network documents. In some implementations, theimages are extracted from documents while in some other implementations,the images are stand alone images, for example, from an imagerepository. In particular, the collection of images can correspond toimages that have been selected by one or more users when provided inresponse to an image search text query.

The image database 102 also includes query phrases and a count ofselections (“clicks”) that each corresponding image in the collectionreceived when identified in response to the query phrase. A query phrasecan be part of or all of a submitted text query where a longer textquery can be segmented into query phrases.

For example, if the query phrase “Eiffel Tower” is submitted as, or partof, a text query to a search system, the search system can identify anumber of image resources as image results. These image results can bepresented, for example, as thumbnail representations of thecorresponding resources. A user can select, e.g., by clicking on with aninput device or tapping on a touchscreen interface, a particularthumbnail representation in order to view the corresponding image. Foreach selected image there can be one or more corresponding query phrasesthat result in the image being identified in search results.

The collection of images from the image database 102 is filtered toidentify a group of images 104 satisfying a particular threshold numberof clicks. For example, the group of images can include only thoseimages from the collection of images that received at least a thresholdnumber of clicks when a representation of the image is presented insearch results.

For each image in the image database 102, metadata is identified andstored in a metadata database 106. The metadata database stores, foreach image, query phrases each having a corresponding count ofselections received (referred to as “clicks” throughout thespecification for convenience though this applies to other types ofselections generally), referrers, and text snippets from the referrers.Referrers are uniform resource locators “URLs” of web pages that containor link to the image. Text snippets can include snippets that describethe web pages themselves or that are physically close to the image. Textsnippets can also include anchor text for links to the image. Other textassociated with the image can also be stored in the metadata database,for example, user-defined labels describing the image.

The group of images 104 and the metadata in the metadata database 106are both used by an indexer 108 to generate a searchable index forimages in the group of images 104. Generating the searchable index isdescribed in greater detail below with respect to FIG. 2.

FIG. 2 is a flow chart showing an example method 200 for generating asearchable index of image content. The index generated by the method 200indexes image descriptors. Each image descriptor is associated with animage. The method 200 can further identify n-grams to associate with oneor more of the images whose descriptors are indexed.

The method 200 can be performed, in some implementations, by one or morecomponents of a system of one or more computing devices, e.g., thesystem component can be the indexer 108 of system 100 shown in FIG. 1.Therefore, for convenience, the example method 200 will be described inreference to a system that performs the method 200.

The system receives 202 a collection of images. The collection of imagescan be generated by collecting images from various network sources,e.g., as described above with respect to the image database 102.

The system uses the collection of images to select 204 a group of imagessatisfying a threshold number of clicks. The number of clickscorresponds to the number of times a representation of the imagepresented as part of image search results was selected by a user inresponse to a text query.

For each of the images in the group of images, the system identifies 206associated metadata. The metadata associated with an image can include,for example, query phrases and the corresponding count total of receivedclicks, referrers, and text snippets from the referrers, as set forthabove with respect to FIG. 1.

Additionally, for each image of the group of images the systemcalculates 208 one or more local descriptors. Each local descriptor foran image represents an abstract description of a local area in an image.

The system can generate local descriptors for a given image by firstdividing the image into multiple regions. The regions can be identifiedregions of interest. A region of interest can be specified by itslocation, scale, and orientation. The regions can then be normalized sothat the normalized regions have a same orientation and size as apre-determined reference region. For example, similarity or affinetransforms can be applied to the regions to normalize them. Inparticular, the normalization can include rotating or rescaling theregion. In some implementations, the local regions are scaled to aspecified reference region that matches generated wavelets, which aredescribed in more detail below.

The system generates one or more wavelets using the dimensions andorientation of the normalized regions. Wavelets are functions thatresolve data into different frequency components and then analyze eachcomponent with a resolution matched to its scale. In someimplementations, Gabor wavelets are used.

The system calculates one or more dot products between each of the oneor more wavelets, respectively, and the normalized regions. The resultsof each dot product provides a local descriptor vector for thecorresponding region of the image. Thus, a local descriptor iscalculated for each region of interest in the image. In someimplementations, the dot product is calculated to retain phaseinformation. The calculated values are used as entries in a localdescriptor vector for the image.

In some implementations, the system concatenates amplitudes of the oneor more dot products to generate the local descriptor vectorrepresenting each region of the image. The use of amplitudes can reduceerror effects caused, for example, by localization errors of regions inthe image. Additionally, the local descriptor vectors generated throughconcatenation of amplitudes can be used to generate a disparity value.Disparity refers to displacement in an image required to obtain anoptional match between local regions of different images. Computation ofa disparity value can, for example, reduce localization errors.

Additionally, in some implementations the local descriptor vectors arenormalized, for example, to maintain constant L-norms. Further, thelocal descriptor vectors can be compressed to reduce the number ofdimensions present in each vector. For example, the dimension of thevectors can be reduced by using principal component analysis. Theselocal descriptor vectors for the group of images can then be used togenerate a searchable index of local descriptor vectors.

The system generates 210 an initial searchable index of the group ofimages using the local descriptor vectors. The initial searchable indexindexes the one or more local descriptor vectors generated for eachimage of the group of images. Additionally, each local descriptorvector, or data associated with the local descriptor vector, canidentify the corresponding image such that for any individual localdescriptor in the index, the source image can be identified.

Various techniques to generate the initial searchable index can be used.A particular indexing technique can be selected such that given a largeset of images and a query image the searchable index returns all imagesin the large set that are visually similar to the query image. The indexalso provides for the similarity of images to be qualified with a scoree.g., a score in the range [0, 1].

For example, the index can be built as follows. For each of the selectedimages, a set of local descriptors is computed. From these descriptors,a searchable index is built. In general, the searchable index, given alarge set of images and a query image, is used to identify images in theset that are visually similar to the query image and to quantify thesimilarity of the returned images.

In some implementations, a distributed kd-tree structure is used for thesearchable index. While the example searchable index described below isbuilt from a kd-tree structure, other types of searchable indexes canalso be used.

A kd-tree is distributed across computing devices by creating 2^(d)sub-trees at a level, or depth, d in the kd-tree. The sub-trees at eachnode at depth d include all of the respective descendants from the node.The nodes at a depth less than d (the remaining “tree stump”) are usedto decide which sub-trees are queried for each descriptor (queryvector).

Given a very large set of N-dimensional vectors to be searched (e.g.,each representing an individual local descriptor from an image), asample subset is drawn from the set and the tree stump (a kd-tree withfixed depth d) is built from the sample. The tree stump is then used topartition the original set of N-dimensional vectors into 2^(d)partitions by assigning every sample vector to the leaf node thatcontains the sample vector. The partitioned kd-trees are built from eachpartition of vectors. In some implementations, each partitioned tress ishosted on one machine.

Once built, this distributed data structure is searched by firstdetermining all leaves on the tree-stump within a certain maximumdistance from the query and then sending the query to the kd-treescorresponding to these leaves for processing. Once results are retrievedfrom all leaves, the results are combined and ranked. This combinationand ranking results in a set of images that are most closely related tothe query.

FIG. 3 is a block diagram of an example distributed kd-treeinfrastructure 300. The distributed kd-tree infrastructure 300 includesa first tree 311 and multiple sub-trees 321-1 . . . 321-m. In theexample infrastructure shown, the tree stump 311 is a kd-tree of depthd, and thus there are 2^(d) sub-trees 321-1 . . . 321-m, i.e., m=2^(d).In some implementations, each sub-tree 321 is configured to be of a sizethat can be stored in a random access memory (RAM) of a single computerdevice 320. Likewise, in some implementations, the tree stump isconfigured to be of a size that can be stored in the RAM of the computerdevice 310.

In general, each leaf node in the sub-tress 321-1 . . . 321-mcorresponds to one element in a dataset 302. For example, for the systemdescribed herein, each leaf node in the sub-trees 321-1 . . . 321-mcorresponds to an N-dimensional query vector of descriptors. The treestump 311 is used to determine which sub-tree(s) 321 a received queryvector is to be sent for processing. The distributed kd-treeinfrastructure 300 can also be used to index other types of data,however.

FIG. 4 is a flow chart showing an example method 400 for constructing adistributed search structure. The method 400 can be performed, in someimplementations, by one or more components of a system including one ormore computing devices, e.g., one or more of the computers of FIG. 3.Therefore, for convenience, the example method 400 will be described inreference to a system that performs the method 400.

The system obtains 402 a set of N-dimensional vectors. Each vectorcorresponds to a local descriptor for an image. In some implementations,for partitioning purposes, N is considered large if a full set of Nvectors cannot be stored in the RAM of a single machine.

From this set, the system selects 404 a smaller number of vectors. Insome implementations, the selection is performed pseudo-randomly orrandomly. In particular, the system can pseudo-randomly select a propersubset of vectors to reduce the likelihood of biasing the resultingtrees stump that is to be built.

The system uses 406 this proper subset of vectors to build the treestump for a kd-tree. The tree stump (a kd-tree with fixed depth d) canbe built by any conventional kd-tree building algorithm. One examplemethod uses splitting planes and point insertion to construct thekd-tree. The system cycles through axes that are used to selectsplitting planes, and inserts points by selecting the median of thepoints that are to descend from a node with respect to their coordinatesin the axis being used to create the splitting plane. Other kd-treebuilding algorithms can also be used.

As the tree stump is limited to d levels, it contain 2^(d) leaf nodes.Each leaf node corresponds to a hyper-rectangle in the M-dimensionalspace.

Once the tree stump is built, the system assigns 408 each of the vectorsin the larger set to a bin corresponding to the leaf node in which thevector lies. The assignment of the vectors to a bin is described below.The assignments of the vectors partitions the set of N vectors into2^(d) bins with approximately equal vector counts. Because the initialsample used to build the tree stump may not exactly reflect the actualdistribution of vectors in the set, the 2^(d) bins typically do not haveexactly equal vector counts.

The system builds 410 a kd-tree from the vectors in each bin. Thekd-trees built from the vectors in each bin are the sub-trees 321 Aswith the tree stump, each sub-tree can be built by any conventionalkd-tree building algorithm. In some implementations, each leaf of eachsub-tree corresponds to only one vector from the dataset 302 (e.g., asingle local descriptor), and each sub-tree 321 is hosted by only onemachine 320.

The structures of the tree stump 311 and the 2^(d) sub-trees 321 definethe distributed kd-tree. In some implementations, each sub-tree isassociated with only one leaf of the tree stump, and thus the structuresof the tree stump and the 2^(d) sub-trees form a complete treestructure.

To find nearest neighbors, the sub-tree in which the query descriptorbelongs and all k−1 sub-trees, with k≦m (the maximum number of sub-treesearches per query), are searched. Each of the k−1 sub-trees are alsosub-trees whose root is on a path where the distance of the querydescriptor to the splitting plane of the node where a “wrong” directionwas taken is below a given threshold. The results are combined from eachtree to return the closest set of nearest neighbors, e.g., the closestset of descriptors.

FIG. 5 is a flow chart showing an example method 500 for an approximatenearest neighbor search on the distributed search structure. The method500 can be performed, in some implementations, by one or more componentsof a system, e.g., using one or more of the computers of FIG. 3.Therefore, for convenience, the example method 500 will be described inreference to a system that performs the method 500. The nearest neighborsearch identifies, for example, images that are visually similar to agiven image.

Given a single M-dimensional query vector for a particular localdescriptor of an image, the system compares 502 the vector against thetree-stump. The comparison identifies all leaves of the tree stump thatmay contain vectors within a certain radius of the query vector. In someimplementations, the comparison is done by a process that is similar tothe assignment of the vectors to the bins, and is described below.

The system transmits 504 the query vector to the machines hosting thesub-trees associated with the identified leaves. For example, thecomputer 310 is in data communication with the computers 320-1 . . .320-m, and transmits the query vector to the computers that correspondto the identified sub-trees 321.

The system compares 506 the query vector against each sub-tree toidentify approximate nearest neighbors (e.g., descriptors), and theapproximate nearest neighbors are returned. In some implementations, thenearest neighbors are found using a recursive nearest neighbor search.However, the recursion can be limited to searching within each sub-tree321 that received the query vector, and thus candidate nearest neighborsare found in parallel in each of the sub-trees 321 that were selected toprocess the query vector.

The system combines 508 the results from all sub-trees for all of thequery vectors of the query image to generate the final identification ofdescriptors that match the query vector.

The process of assigning vectors to bins (“binning vectors”) is similarto the process of selecting sub-tress to process a received queryvector. When binning a vector, the tree stump is traversed and only oneleaf node of the tree stub 311 is identified, as that is the leaf nodeassociated with vectors that will be used to generate the eventualsub-tree 321 that “descends” from the leaf node. When selectingsub-trees to process a received query vector, the tree stump istraversed in a similar manner and a list of one (or more) sub-trees isgenerated, and the query vector is then sent to those identified subtrees.

The process of binning vectors is, in some implementations, a series ofdepth first searches on the tree stump 311. Each search yields one entryin a result set S. During each search, the forks not taken are pushedonto a priority queue. Each subsequent search begins at the fork withthe highest priority, which is the closest “near miss” in terms ofdescriptor distance (or other vector element) encountered thus far.

In particular, the parameters for the process include an input vectorv[i], a tree stump with d levels, a split value m[j], and a splitdimension x[j], where j=1 . . . 2^(d). For any node j, the child nodesof a node indexed by j are j(2) and j(2+1).

A maximum number of sub-trees L is also defined. L is set to 1 whenbinning, and L is set to a value greater than 1 when selecting sub-treesto process a query vector. Additionally, a maximum distance dist_max isalso set, and defines the maximum distance between the space occupied bya sub-tree and the input vector v[i].

For binning, the process outputs an index of a sub-tree 321, e.g.,S={s}, where s is one of 1 . . . 2^(d). For searching an input vector,the process outputs a list of sub-tree 321 indexes, S={s₁ . . . s_(k)},wherein 1<k<2^(d).

An empty priority queue q of indices is defined, and which will holdindices sorted by increasing distance. An empty set S is defined, and jis set initially to 1.

While the number of elements in S is less than L the process willprocess the input vector v[i]. While j is less than 2^(d), a distancevalue dist is determined, where dist=v[x[j]]−m[j]. In someimplementations, this distance is the signed distance to the separationplane at the node j.

If the distance value dist is less than a threshold (for example, ifdist is negative when the threshold is zero), and if the absolute valueof the distance value dist is less than the maximum distance dist_max,then the child node corresponding to j*2+1 is inserted into the priorityqueue with a corresponding distance of |dist|.

Conversely, if the distance value dist is not less than the threshold(for example, if dist is positive when the threshold is zero), and ifthe absolute value of the distance value dist is less than the maximumdistance dist_max, then the child node corresponding to j*2 is insertedinto the priority queue with a corresponding distance of |dist|, and thevalue of j is incremented by 1.

The next value dist is determined, and the process continues until j isequal to or greater than 2^(d). When this condition occurs, the currentnode indexed by j is added to the list index of sub-trees, i.e., thevalue of j−2^(d) is inserted into the set S. If the queue is empty orthe maximum sub-tree limit L is hit, the process ends, otherwise j isset to the top value pulled from priority queue q and the processcontinues.

This process described above for assigning vectors to bins can bewritten in pseudocode, for example, as:

q <− empty priority queue of indices, sorted by increasing distance S <−empty set j <− 1 // i.e. root node of tree stump) while number ofelements in S < I do while j < 2{circumflex over ( )}d do dist <−v[d[j]] − m[j] // signed distance to separation plane at node j j <− j *2 // first child node if dist < 0 // inserts other child node intoqueue, continues with j if |dist| < dist_max insert j+1 into priorityqueue at distance |dist| else // inserts this (j) child node into queue,continues with j+1 if |dist| < dist_max insert j into priority queue atdistance |dist| j <− j+1 insert j − 2{circumflex over ( )}d into S //inserts shard index into result set. if q is empty end  // obtains nextnode to start with from queue, i.e. node in queue with smallestdistance. j <− top of q remove top of q end

As shown in FIG. 2, the system identifies 212 as set of matching imagesfor each image of the group of images using the initial searchableindex. The matches are qualified by a score that is correlated to avisual similarity between matching images. In some implementations, thescore has a value in the range [0, 1]. Various techniques foridentifying similarity can be used. One technique for identifyingsimilar images is described in David G. Lowe, “Object Recognition fromLocal Scale-Invariant Features,” iccv, vol. 2, pp. 1150, SeventhInternational Conference on Computer Vision (ICCV′99)—Volume 2, 1999.

In general, this technique includes finding a number of nearestneighbors for each of the local descriptors of a query image (e.g.,according to the index), for example, as described above with referenceto FIG. 5. The nearest neighbors are then binned by the indexed image towhich they belong. For each indexed image that yielded at least threesuch nearest neighbors in the process, the system examines the geometricrelation of the local areas corresponding to the descriptors in thequery image and the local areas corresponding to the found nearestneighbors in the index image. Essentially, the process establishesarea-to-area correspondences between query and indexed image. The systemlooks for multiple correspondences (e.g., more than a threshold numberof correspondences, for example, three) that agree (using agglomerativeclustering) on the same geometric relation. The number ofcorrespondences found in this way, relative to the total number ofdescriptors in the query image, indexed image, and total indexdetermines the score based on an inverse-Bayes formula.

The matching technique can be modified from those described in Lowe inthat the local descriptors described above are used instead of thedescriptors described in Lowe. Additionally, in some implementations,agglomerative clustering on a distance metric between affine transformsis used in place of the inverse Hough transform.

The system uses the group of images and their respective matches togenerate 214 an undirected graph. The undirected graph has images fromthe group of images as vertices and matches between images as edgesbetween images. The edges can be weighted according to the match scores(e.g., each edge has a weight in the range [0, 1]).

The system processes 216 the graph to determine pairwise image affinityand to determine a final image selection. The pairwise image affinity iscalculated by identifying a maximum affinity path between any two imagesin the graph within a specified number of hops (edges) apart (e.g.,three hops). The image affinity is defined as the product of the edgeweights (i.e., match scores) along the path. For example, if thedistance between image i and image j is two hops, the first hop along anedge having a weight of 0.6 and the second hop having a weight of 0.2,then the affinity between images i and j is equal to (0.6)(0.2)=0.12.Identifying a maximum affinity path between images is analogous to ageneral shortest path problem for an undirected graph. The maximumaffinity path can be solved in a similar manner to the general shortestpath problem, for example by using Dijkstra's algorithm.

The system also processes 216 the graph to identify the final imageselection. The images in the final image selection are those that willbe used to generate a new searchable index. Each image in the graphincludes a selection flag, s_(i). The selection flag has a value of 0 or1, where a value of 1 means the image is included in the finalselection. In some implementations, the selection flag for each image isinitially set to 1. As described above, the edges of the graph each havea weight w_(ij). Thus, the weights w_(ij) connect each image i with animage j. Additionally, the weight of an image with respect to itself,w_(ii) can be defined as equal to 1.

A yield over the entire graph can be calculated where the yield Y isdefined by: Y:=Sum_(i)(max_(j)(s_(j)) (w_(ij)))−(selection cost)(s_(i)).

Maximizing the yield results in the selection of images that have themaximum resolving power for searches. For example, the selection of aset of images that most likely will be responsive to any possible usersearch. Specifically, if a particular image is not selected, theaddition to the yield is equal to the maximum edge value of that imageto an image that is selected. However, if a particular image isselected, the addition to the yield is equal to 1-selection cost. Theselection cost is a predetermined penalty applied to the yield wheneveran image is selected (e.g., added to the set of final images that willbe included in the searchable index).

For example, if two pictures of the Eiffel Tower at night are connectedin the graph with high weight, having both in the final set of imagesdoes not have much effect on whether responsive images can be returnedto a user—a user will likely be happy with either image, given thestrong visual similarity. Therefore, in some implementations, only oneof them is included in the final image selection.

The yield is maximized iteratively to determine the optimal value foreach selection flag s_(i). In each iteration and for each node, Δ_(yi)(the change in the yield due to toggling node i) is calculated. The nodei is toggled with a probability of (Δ_(yi))(change_probability_factor)if Δ_(yi) is positive. The change_probability_factor is a constant witha predetermined value. The constant is selected to convert the value forΔ_(yi) into a probability. The iterations terminate when either aspecified maximum number of iterations is reached or when no nodes withpositive Δ_(yi) exist (e.g., no change to the selection flag for animage increasing the yield for the graph). The s_(i) values for theoptimal yield are stored and include the selection flags for the imagesin the collection of images, where those images having a selection flagvalue of 1 are included in the final image selection.

As shown in FIG. 2, the system calculates 218 a set of n-grams, havingcorresponding n-gram scores, for each image using the image affinity andthe image metadata. In particular, for each image, a set of n-grams andassociated n-gram scores are calculated from n-grams for visuallysimilar images using the image metadata and image affinity.

For each image, a set of n-grams is computed individually. An n-gram isa sequence of n tokens, for example, n-words. The set of n-grams caninclude n-grams of different orders (number of tokens) from 1 to N. Ann-gram exists for an image, for example, if that n-gram appears in atleast one query phrase having the specified threshold number of clicksor if the n-gram appears in a sufficient number of image referrers (webpages that contain the image). The click and referrer information ispart of the metadata identified for each image of the group of images,as described above. Other sources of n-grams can also be used, forexample, n-grams can be derived from labels associated with an image orother metadata for the image.

The image affinity between images i and j, calculated above, is writtenas a_(ij). A count of a number of images within a neighborhood of animage i can be defined as N_(i):=Sum_(j)(a_(ij)) where the neighborhoodis defined by images having a non-zero affinity with image i. Thus, theneighborhood includes images j within a specified number of hops fromimage i where each affinity a_(ij) is non-zero. For each n-gram k, a‘count’ C of images is calculated for images identified as in theneighborhood that are associated with the n-gram:C _(ik):=Sum_(j has n-gram k)(a _(ij)).

The score of an n-gram is then defined as: w_(ik):=max(0,(C_(ik)−1)/C_(i)). The score for an n-gram is non-zero if at least twoimages in the neighborhood are associated with (e.g., ‘have’) then-gram. The n-grams having a weight of zero can be filtered out asspurious (i.e. unconfirmed) n-grams. The score for a given n-gram willconverge toward 1 as the number of images in the neighborhood as well asthe fraction of images that ‘have’ the n-gram increases. In addition,the score for an n-gram k can be positive even if image i was notinitially associated with the n-gram, for example, when images visuallysimilar to the image are associated with the n-gram. Together, thisensures that images have as many high quality (i.e. confirmed) n-gramsas possible.

A final searchable index is generated 220 from the images in the finalimage selection (e.g., as described above using a kd-tree). The finalsearchable index is an index of local descriptors, for example, asdescribed above with reference to FIGS. 3-5. The system can alsomaintain data associating images whose descriptors are in the finalsearchable index with the n-grams identified for the image. The newsearchable index can be used to identify, in response to an image query,one or more images matching local descriptors in the index. Once theimages are identified, the n-grams can be retrieved according to thestored data. The final searchable index and n-gram association data canbe used to respond to image queries, for example, as described belowwith respect to FIGS. 6-8.

FIG. 6 illustrates an example search system 614 for providing searchresults, e.g., images, documents, videos, multimedia content, etc.relevant to submitted queries as can be implemented in an Internet, anintranet, or other client and server environment. The search system 614is an example information retrieval system. In some implementations, thesearch system 614 is specialized to search particular kinds of content,e.g., images. In some other implementations, the search system 614searches content in general.

A user 602 interacts with the search system 614 through a client device604. For example, the client device 604 can be or can include a computer(e.g., a personal computer, a mobile phone, etc.) coupled to the searchsystem 614 through a wired or wireless local area network (LAN) or widearea network (WAN), e.g., the Internet. In some implementations, thesearch system 614 and the client device 604 are both implemented in thesame machine. For example, a user can install a desktop searchapplication on the client device 604. The client device 604 willgenerally include a random access memory (RAM) 606 and a processor 608.

A user 602 submits a query 610 to a search engine 630 within the searchsystem 614. The query can be, for example, an image query. An imagequery is a query that is an image. When the user 602 submits a query610, the query 610 is transmitted through a network to the search system614. The search system 614 can be implemented as, for example, computerprograms running on one or more computers in one or more locations thatare coupled to each other through a network. In some implementations,the search system 614 includes an index database 622 and a search engine630. The index database 622 can be built, for example, using thetechniques described above with reference to FIGS. 2-5 and with regardto the indexer 108 of FIG. 1. Alternatively, or additionally, the indexdatabase 622 can be a conventional search database built, for example,using conventional techniques. The search system 614 responds to thequery 610 by generating search results 628, which are transmittedthrough the network to the client device 604 in a form that can bepresented to the user 602 (e.g., in a search results web page to bedisplayed in a web browser running on the client device 604). Forexample, the search results can include images, documents, and otherresults responsive to the user query 610.

When the query 610 is received by the search engine 630, the searchengine 630 identifies search results that match the query 610. When aquery is an image, the search engine 630 can identify search resultsthat match the query 610 by identifying visually similar images for theimage query, for example, as described above with reference to FIGS. 2-5and returning one or more of the images as search results. Alternativelyor additionally, the search engine can identify a textual querycorresponding to the image and identify search results responsive to thetextual query, for example, using conventional search techniques.Identifying the textual query is described in more detail below, withreference to FIGS. 7-8.

The search engine 630 will generally include an indexing engine 620 thatindexes content found by the search system 614, for example, documentsfound while crawling the Internet, an index database 322 that stores theindex information, and a ranking engine 652 (or other software) to rankthe search results that match the query 610, for example, according to aresult score associated with each result by the engine 630. The resultscore can be a query-independent measure of the quality of the result, aquery-specific measure of how well the result matches the query, or ascore derived from both a query-independent measure and a query specificmeasure. The search engine 630 transmits the search results 628 throughthe network to the client device 604 for presentation to the user 602.

FIG. 7 is a flow chart of an example method 700 for obtaining searchresults responsive to an image query received from a user. Forconvenience, the example method 700 will be described in reference to asystem including one or more computing devices that performs the method700. The system can be, for example, the search system 614 describedabove with reference to FIG. 6.

The system receives 702 an image as a query. The system can receive theimage, for example, when a user submits the image as a search query. Forexample, a user can submit an image captured with a camera on a device.In some implementations, the device is a mobile device.

The system derives 704 a textual query corresponding to the image. Thetextual query is an n-gram that is considered a description of theimage. An example method for deriving an n-gram that describes an imageis described below, with reference to FIG. 8.

The system obtains 706 search results responsive to the textual query.The system identifies responsive search results that best match thetextual query, for example, using conventional techniques.

In some implementations, once the system obtains the search results, thesystem presents the search results to the user, for example, asdescribed above with reference to FIG. 6. In some implementations, thesystem also obtains search results that are visually similar to theimage query and presents those search results with the search resultsresponsive to the textual query. The system can obtain the visuallysimilar search results, for example, as described above with referenceto FIGS. 2-5.

FIG. 8 is a flow chart of an example method 800 for selecting a textualdescription of an image. For convenience, the example method 800 will bedescribed in reference to a system including one or more computingdevices that performs the method 800. The system can be, for example,the search system 614 described above with reference to FIG. 6. Thesystem obtains 802 a group of similar images for a particular image. Thesystem can obtain the group of similar images, for example, by queryingan index that associates images with descriptors, for example thedescriptor index described above with respect to FIG. 2. Querying theindex can include submitting a query vector for each local descriptor ofthe query image. Additionally, each similar image in the index isassociated with one or more n-grams. The association can be stored, forexample, in the final searchable index described above with reference toFIG. 2. In some implementations, each n-gram has an image-specific scorefor each image with which it is associated. The n-grams and scores canbe identified for the images, for example, as described above withrespect to FIG. 2.

The system determines 804 an overall score for one or more of then-grams. The system determines the overall score by combining theimage-specific scores for each n-gram.

For example, the system can determine the overall score for a givenn-gram k according to the following formula:

${{{overall}\mspace{14mu}{score}_{k}} = {c^{N - 1} \times {\sum\limits_{{score}_{k,i} > {threshold}}^{\;}\;\frac{{score}_{k,i}}{{number}\mspace{14mu}{of}\mspace{14mu}{images}}}}},$where N is the order of the n-gram k (e.g., the number of words in then-gram), the score_(k,i) is the image specific score for n-gram k andimage i, and the number of images is the number of images in the groupof similar images. The threshold is determined, for example, empiricallyto filter out scores that are considered too low to be counted. Thevariable c is a constant chosen empirically, for example, to increasethe score of higher order n-grams, in effect, boosting n-grams thatinclude more words and thus are less likely to occur a-priori.

In some implementations, the system filters the n-grams, e.g., bysetting the score of undesired n-grams to a predetermined value such aszero. For example, n-grams that contain only non-alphabeticalcharacters, n-grams that begin or end with a stop word, or n-grams thatcontain a word on a blacklist can have their scores set to zero. Thestop words are words that the system considers too general to addmeaning to user queries. Example stop words include “the,” “a,” “and,”and file extensions, e.g., “.jpg.” The system can determine whether ann-gram contains a stop word, for example, by comparing the terms of then-gram to a predefined list of stop words. The blacklist can include,for example, terms that have been predetermined to be offensive, such assexual terms or derogatory terms.

The system selects 806 one of the n-grams as a textual description ofthe particular image. The system selects the selected n-gram accordingto the overall scores for the n-grams.

For example, in some implementations, the system considers the n-gramsin order of increasing N. The system beings by considering the overallscores for all n-grams for the group of similar images where N=1. Thesystem selects the n-gram that has the highest score of all of then-grams where N=1 as the current best n-gram.

The system then considers other highest scoring n-grams for each n-gramorder N in increasing order. If the highest scoring n-gram has anoverall score higher than the current best n-gram, the system selectsthe highest scoring n-gram for the current n-gram order as the currentbest n-gram.

If no n-gram for the current n-gram order has a score higher than theoverall score for the current best n-gram, the system determines whetherany superstrings of the current best n-gram are included in the n-gramsfor the current value of N. If the n-grams for the current value of Ninclude one or more superstrings of the current best n-gram, the systemdetermines whether the overall score for the highest scoring superstringis above an acceptance threshold. If so, the superstring becomes thecurrent best n-gram. Otherwise, the current best n-gram is unchanged. Insome implementations, when the system selects the superstring as thecurrent best n-gram, the system associates the score of the substring ofthe superstring with the superstring as the score of the superstring.

Once the n-grams for all possible values of N have been considered, thesystem selects the current best n-gram as the textual description of theparticular image.

For example, suppose the system has identified the following n-grams fora group of similar images, along with the following overall scores foreach n-gram:

“Eiffel”: 0.7

“Paris”: 0.6

“Trip”: 0.2

“Landmark”: 0.1

“Eiffel Tower”: 0.8

“Paris Trip”: 0.3

“of Paris”: 0.25

“Paris Tower”: 0.2

“Landmarks of Paris”: 0.2

“from Paris Trip”: 0.18

“Eiffel Tower at”: 0.15

“Pictures from Paris Trip”: 0.12

“Eiffel Tower at Night” 0.11.

In this example, the system can determine the textual description of theparticular image as follows. The system first determines that the best1-gram is the phrase “Eiffel” with an overall score of 0.7. Therefore,the system selects “Eiffel” as the current best n-gram with score 0.7.The system then determines that the phrase “Eiffel tower” is the best2-gram with a score of 0.8 and selects “Eiffel tower” as the currentbest n-gram with a score of 0.8.

The system then determines that the highest scoring 3-gram is “Landmarksof Paris” with a score of 0.2. The system determines that 0.2 is lessthan the overall score of the current best n-gram (0.8). Therefore, thesystem does not update the current best n-gram to be “Landmark ofParis.” However, the system also determines that “Eiffel tower at,” asuperstring of the current best n-gram “Eiffel tower” has a score of0.15 which is above a specified acceptance threshold. Therefore, thesystem selects the 3-gram “Eiffel tower at” as the current best n-gramwith a score of 0.8 (corresponding to the score for “Eiffel tower.”).

The system then determines that the best 4-gram is the phrase “Picturesfrom Paris Trip” with a score of 0.12; however, this is less than thescore of the current best n-gram (0.8). Therefore, the system does notupdate the current best n-gram to be “Pictures from Paris Trip.”However, the system also determines that “Eiffel tower at night” is asuperstring of the current best n-gram “Eiffel tower at” and has a scoreof 0.11 which is above the acceptance threshold. Therefore, the systemselects the 4-gram “Eiffel tower at night” as the new current bestn-gram with a score of 0.8.

In this example, the system then selects “Eiffel tower at night” as thetextual description of the particular image, because it is the currentbest n-gram after all n-grams have been considered.

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments of the subject matterdescribed in this specification can be implemented as one or morecomputer programs, i.e., one or more modules of computer programinstructions, encoded on computer storage medium for execution by, or tocontrol the operation of, data processing apparatus. Alternatively or inaddition, the program instructions can be encoded on anartificially-generated propagated signal, e.g., a machine-generatedelectrical, optical, or electromagnetic signal, that is generated toencode information for transmission to suitable receiver apparatus forexecution by a data processing apparatus. A computer storage medium canbe, or be included in, a computer-readable storage device, acomputer-readable storage substrate, a random or serial access memoryarray or device, or a combination of one or more of them. Moreover,while a computer storage medium is not a propagated signal, a computerstorage medium can be a source or destination of computer programinstructions encoded in an artificially-generated propagated signal. Thecomputer storage medium can also be, or be included in, one or moreseparate physical components or media (e.g., multiple CDs, disks, orother storage devices).

The operations described in this specification can be implemented asoperations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, object, orother unit suitable for use in a computing environment. A computerprogram may, but need not, correspond to a file in a file system. Aprogram can be stored in a portion of a file that holds other programsor data (e.g., one or more scripts stored in a markup languagedocument), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,sub-programs, or portions of code). A computer program can be deployedto be executed on one computer or on multiple computers that are locatedat one site or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. The essential elements of a computer area processor for performing actions in accordance with instructions andone or more memory devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom or transfer data to, or both, one or more mass storage devices forstoring data, e.g., magnetic, magneto-optical disks, or optical disks.However, a computer need not have such devices. Moreover, a computer canbe embedded in another device, e.g., a mobile telephone, a personaldigital assistant (PDA), a mobile audio or video player, a game console,a Global Positioning System (GPS) receiver, or a portable storage device(e.g., a universal serial bus (USB) flash drive), to name just a few.Devices suitable for storing computer program instructions and datainclude all forms of non-volatile memory, media and memory devices,including by way of example semiconductor memory devices, e.g., EPROM,EEPROM, and flash memory devices; magnetic disks, e.g., internal harddisks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks. The processor and the memory can be supplemented by, orincorporated in, special purpose logic circuitry.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor, for displaying information to the user and akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide for interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back-end component,e.g., as a data server, or that includes a middleware component, e.g.,an application server, or that includes a front-end component, e.g., aclient computer having a graphical user interface or a Web browserthrough which a user can interact with an implementation of the subjectmatter described in this specification, or any combination of one ormore such back-end, middleware, or front-end components. The componentsof the system can be interconnected by any form or medium of digitaldata communication, e.g., a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), an inter-network (e.g., the Internet), andpeer-to-peer networks (e.g., ad hoc peer-to-peer networks).

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someembodiments, a server transmits data (e.g., an HTML page) to a clientdevice (e.g., for purposes of displaying data to and receiving userinput from a user interacting with the client device). Data generated atthe client device (e.g., a result of the user interaction) can bereceived from the client device at the server.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results. In addition, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

What is claimed is:
 1. A method performed by data processing apparatus,the method comprising: obtaining a group of similar images for aparticular image, wherein each similar image is associated with one ormore n-grams; determining an overall score for each of the n-grams,wherein for each n-gram the overall score is based at least in part on:i) a combination of image specific scores for the n-gram for images inthe group of similar images and ii) the number of similar images in thegroup of similar images; and selecting one of the n-grams as a textualdescription of the particular image according at least in part on theoverall scores for the n-grams and the respective n-gram order of eachof the n-grams representing a number of tokens in the n-gram.
 2. Themethod of claim 1, wherein obtaining a group of similar images for aparticular image comprises querying an index that associates images withdescriptors.
 3. The method of claim 1, further comprising storing adatabase associating each image with one or more n-grams; using thedatabase to determine an n-gram associated with a received image query;and using the n-gram to obtain one or more search results.
 4. The methodof claim 1, wherein each image-specific score for a particular n-gram iscalculated based on image affinity calculated between similar imagesassociated with the n-gram.
 5. The method of claim 1, wherein theoverall score for an n-gram is based on a sum of the image-specificscores for the n-gram divided by a total number of images associatedwith the n-gram.
 6. The method of claim 1, further comprising filteringone or more of the n-grams according to a blacklist of terms.
 7. Themethod of claim 1, further comprising filtering one or more of then-grams to remove n-grams that contain stop words.
 8. The method ofclaim 1, wherein selecting one of the n-grams comprises: selecting acurrent best n-gram from one or more 1-grams in the n-grams, where thecurrent best n-gram has a highest overall score of all of the one ormore 1-grams in the n-grams; considering each other highest overallscoring n-gram of all of the n-grams at each order of n-grams greaterthan 1 in increasing order, and determining, for each highest overallscoring n-gram at a given order whether to update the current bestn-gram to be the highest overall scoring n-gram of the order, where thecurrent best n-gram is updated when the highest overall scoring n-gramof the order has a higher overall score than a score associated with thecurrent best n-gram or when the highest overall scoring n-gram is asuperstring of the current best n-gram and has a score that satisfies anacceptance threshold.
 9. The method of claim 8, further comprisingupdating the current best n-gram to be the determined highest overallscoring n-gram of the order when it is determined that the current bestn-gram should be updated.
 10. The method of claim 9, wherein updatingthe current best n-gram further comprises associating a score with theupdated current best n-gram, wherein the score is the overall score ofthe updated current best n-gram when the updated current best n-gram hasan overall score that is higher than the score associated with theprevious best n-gram, and otherwise is the score of the previous bestn-gram.
 11. A system, comprising: one or more computers operable toperform operations comprising: obtaining a group of similar images for aparticular image, wherein each similar image is associated with one ormore n-grams; determining an overall score for each of the n-grams,wherein for each n-gram the overall score is based at least in part on:i) a combination of image specific scores for the n-gram for images inthe group of similar images and ii) the number of similar images in thegroup of similar images; and selecting one of the n-grams as a textualdescription of the particular image according at least in part on theoverall scores for the n-grams and the respective n-gram order of eachof the n-grams representing a number of tokens in the n-gram.
 12. Thesystem of claim 11, wherein obtaining a group of similar images for aparticular image comprises querying an index that associates images withdescriptors.
 13. The system of claim 11, further operable to performoperations comprising storing a database associating each image with oneor more n-grams; using the database to determine an n-gram associatedwith a received image query; and using the n-gram to obtain one or moresearch results.
 14. The system of claim 11, wherein each image-specificscore for an n-gram is based on image affinity calculated betweensimilar images associated with the n-gram.
 15. The system of claim 11,wherein the overall score for an n-gram is based on a sum of theimage-specific scores for the n-gram divided by a total number of imagesassociated with the n-gram.
 16. The system of claim 11, further operableto perform operations comprising filtering one or more of the n-gramsaccording to a blacklist of terms.
 17. The system of claim 11 furtheroperable to perform operations comprising filtering one or more of then-grams to remove n-grams that contain stop words.
 18. The system ofclaim 11, wherein selecting one of the n-grams comprises: selecting acurrent best n-gram from one or more 1-grams in the n-grams, where thecurrent best n-gram has a highest overall score of all of the one ormore 1-grams in the n-grams; considering each other highest overallscoring n-gram of all of the n-grams at each order of n-grams greaterthan 1 in increasing order, and determining, for each highest overallscoring n-gram at a given order whether to update the current bestn-gram to be the highest overall scoring n-gram of the order, where thecurrent best n-gram is updated when the highest overall scoring n-gramof the order has a higher overall score than a score associated with thecurrent best n-gram or when the highest overall scoring n-gram is asuperstring of the current best n-gram and has a score that satisfies anacceptance threshold.
 19. The system of claim 18, further comprisingupdating the current best n-gram to be the determined highest overallscoring n-gram of the order when it is determined that the current bestn-gram should be updated.
 20. The system of claim 19, wherein updatingthe current best n-gram further comprises associating a score with theupdated current best n-gram, wherein the score is the overall score ofthe updated current best n-gram when the updated current best n-gram hasan overall score that is higher than the score associated with theprevious best n-gram, and otherwise is the score of the previous bestn-gram.
 21. A non-transitory computer-readable storage medium storinginstructions that, when executed by data processing apparatus cause thedata processing apparatus to perform operations comprising: obtaining agroup of similar images for a particular image, wherein each similarimage is associated with one or more n-grams; determining an overallscore for each of the n-grams, wherein for each n-gram the overall scoreis based at least in part on: i) a combination of image specific scoresfor the n-gram for images in the group of similar images and ii) thenumber of similar images in the group of similar images; and selectingone of the n-grams as a textual description of the particular imageaccording at least in part on the overall scores for the n-grams and therespective n-gram order of each of the n-grams representing a number oftokens in the n-gram.